Gross anatomical examination of the ligaments crossing the human ankle and subtalar joints and clinical experience with their injuries point to their major role as joint stabilizers. However, there is no quantitative data to provide precise information about the specific function of each ligament. In the present project, it is proposed to investigate both experimentally and analytically the mechanics of the human ankle and subtalar joints in order to (1) provide a quantitative description of the function of the ligaments in stabilizing the joints during human motion; (2) determine the effect of ligamentuous damage on the mechanical properties of the joints; and (3) identify critical dynamic conditions, occurring during human motion, which may lead to overloading of ligaments. Accomplishment of these goals will provide the basis for (1) the development of quantitative diagnostic techniques for identification of damaged ligaments and for the assessment of the degree of severity of ligamentuous injuries and (2) the improved design of protective devices to minimize the incidences of ligament injuries during high risk activities. To achieve the short term goals of the project, the following procedure will be adopted. Data on the geometrical and mechanical properties of the human ankle and subtalar joints will be obtained from 40 fresh human cadaveric legs. These data will be used to develop a model of the joints. The effect of ligament damage will then be studied. The ligaments will be sequentially disconnected from the cadaveric specimen and following the removal of each ligament, the response of the joints to applied forces and moments will be determined. The same conditions will be simulated on the model and the experimental and analytical results will be compared. Data on the dynamics of the human ankle and subtalar joints during various activities will then be obtained from 20 healthy human subjects. These data will be used as input to the model and through a dynamic analysis performed on it, the forces acting at the joints will be determined. The forces developed by the ligaments will be compared to other forces at the joints. From this comparison, the stabilizing role of each ligament during human motion will be determined.